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Yeung Man Hau. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (leaves 52-53). / Text in English; abstract also in Chinese. / Yeung Man Hau. / Chapter Chapter 1 --- Introduction / Chapter 1. --- Background of solidification --- p.1 / Chapter 1.1 --- The driving force for solidification / Chapter 1.2 --- Capillarity effect (or Gibbs-Thomson effect) / Chapter 2. --- Nucleation --- p.3 / Chapter 3. --- Growth --- p.4 / Chapter 3.1 --- Constrained growth and unconstrained growth / Chapter 3.2 --- Directional solidification / Chapter 4. --- Growth of pure substances --- p.6 / Chapter 4.1 --- Metals / Chapter 4.2 --- Stability of planar S/L interface / Chapter 4.3 --- Non-metals / Chapter 5. --- Solidification of single-phase binary alloys --- p.7 / Chapter 5.1 --- Equilibrium solidification / Chapter 5.2 --- Constitutional undercooling / Chapter 5.3 --- Stability of planar S/L morphology / Chapter 5.4 --- Minimum scale of perturbation in directional growth / Chapter 5.5 --- Development of growth morphology / Chapter 5.6 --- Growth rate of cell/dendrite tip / Chapter 5.7 --- Arm spacing and coarsening / Chapter 6. --- Solidification of binary eutectic alloys --- p.11 / Chapter 6.1 --- Classification / Chapter 6.2 --- Growth of lamellar eutectics / Chapter 6.3 --- Stability of planar morphology / Chapter 6.4 --- Coupled zone (Competitive growth of eutectic and dendrites) / Chapter 6.5 --- Off-eutectic solidification / Chapter 7. --- Solidification of ternary eutectic alloys --- p.14 / References --- p.16 / Figures --- p.17 / Chapter Chapter 2 --- Experimental Methods / Chapter 1. --- Fused silica tube cleaning --- p.37 / Chapter 2. --- Alloy preparation --- p.37 / Chapter 3. --- Undercooled specimen preparation --- p.38 / Chapter 4. --- Specimen examination --- p.38 / Chapter 5. --- TEM sample preparation --- p.39 / References --- p.40 / Figures --- p.41 / Chapter Chapter 3 --- Solidification of Undercooled Molten Pd60 .5Cu25Si14.5 Alloy / Chapter 1. --- Introduction --- p.44 / Chapter 2. --- Experimental --- p.46 / Chapter 3. --- Results --- p.46 / Chapter 3.1 --- Thermal profiles / Chapter 3.1.1 --- Temperature-time chart plotter (plotter) / Chapter 3.1.2 --- Differential thermal analysis (D TA) / Chapter 3.2 --- Microstructures / Chapter 3.2.1 --- Effect of undercooling on the microstructure / Chapter 3.2.2 --- Effect of quenching after 1st exothermic peak on the microstructure / Chapter 3.2.3 --- Effect of annealing at the onset temperature of 1st exothermic peak on the microstructure / Chapter 3.2.4 --- Effect of using slower cooling rate on the microstructure / Chapter 4. --- Discussions --- p.50 / Chapter 5. --- Conclusion --- p.51 / References --- p.52 / Figures --- p.54

Solidification from the melt is an essential step in nearly all conventional processes to produce bulk materials for industrial applications. Rapid quenching from the liquid state at cooling rates of 102 to 106K/s or higher has developed into a new technology for processing novel materials. InxGa1 - xSb a ternary III-V compound semiconductor was synthesized by using the rapid spinning cup (RSC) technique. Several compositions of these alloys were batched and cast into ingots in evacuated sealed quartz tubes. These ingots were then melted and ejected onto a rapidly rotating copper disk. This resulted in the generation of flakes or powders depending on the rpm of the disk. Microstructural characterization of the flakes and powders was performed using XRD, SEM and TEM. Efforts were also made to measure the bulk resistivity of the annealed flakes to see the effect of annealing on ordering of the phases.

by Chan Kim Wai. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1997. / Includes bibliographical references. / by Chan Kim Wai. / Chapter Chapter I --- Introduction / Chapter 1.1 --- Rapid solidification / Chapter 1.1.1 --- Rapid quenching --- p.1-1 / Chapter 1.1.2 --- Undercooling --- p.1-2 / Chapter 1.2 --- Grain refinement / Chapter 1.2.1 --- What is grain refinement? --- p.1-5 / Chapter 1.2.2 --- Previous results in grain refinement / Chapter 1.2.2.1 --- Pure metals (or dilute alloys) --- p.1-5 / Chapter 1.2.2.2 --- Alloys --- p.1-9 / Chapter 1.2.2.3 --- Semiconductor --- p.1-10 / Chapter 1.2.3 --- Critical crystal growth velocity V* --- p.1-11 / Chapter 1.2.4 --- Proposed models to grain refinement / Chapter 1.2.4.1 --- Dynamic nucleation and cavitation --- p.1-12 / Chapter 1.2.4.2 --- Remelting (melt-back) --- p.1-14 / Chapter 1.2.4.3 --- Interdendritic fluid flow --- p.1-15 / Chapter 1.2.5 --- Volumetric manifestation of grain refinement --- p.1-15 / Chapter 1.3 --- Aim of this project --- p.1-16 / References / Figures / Chapter Chapter II --- Experimental / Chapter 2.1 --- Pure palladium / Chapter 2.1.1 --- Sample preparation and procedure --- p.2-1 / Chapter 2.1.2 --- Limitation and choice of flux --- p.2-2 / Chapter 2.1.3 --- High temperature furnace --- p.2-3 / Chapter 2.1.4 --- Measurement of specific volume / Chapter 2.1.4.1 --- Theory --- p.2-4 / Chapter 2.1.4.2 --- Setup --- p.2-5 / Chapter 2.1.5 --- Observing internal morphology --- p.2-5 / Chapter 2.2 --- Palladium with insoluble impurity / Chapter 2.2.1 --- Choice of insoluble impurities --- p.2-6 / Chapter 2.2.2 --- Sample preparation --- p.2-7 / References / Figures / Chapter Chapter III --- Results and Discussion / Results / Chapter 3.1 --- Pure palladium / Chapter 3.1.1 --- Specific volume --- p.3-1 / Chapter 3.1.2 --- Grain structure and internal voids --- p.3-2 / Chapter 3.2 --- Palladium with insoluble impurity / Chapter 3.2.1 --- Pinning effect of insoluble impurities --- p.3-3 / Chapter 3.2.2 --- Pd-Ni-S system / Chapter 3.2.2.1 --- Grain refinement in Pd99.9Ni-S)0.1 --- p.3-4 / Chapter 3.2.2.2 --- Change of ΔT* with addition of sulfur --- p.3-5 / Chapter 3.2.2.3 --- Internal voids --- p.3-5 / Discussion / Chapter 3.3 --- Dynamic nucleation of Pd-Ni-S system --- p.3-6 / Chapter 3.4 --- Void formation of pure palladium and Pd-Ni-S --- p.3-6 / Chapter 3.5 --- Grain refinement and specific volume --- p.3-7 / Reference / Figures

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The main aim of this work is to investigate heterogeneous nucleation of solidification of metals and alloys by a combination of differential scanning calorimetry and transmission electron microscopy using a newly modified entrained particle technique. Attention is focused on investigating (a) heterogeneous nucleation of Cd, In and Pb particle solidification by Al in rapidly solidified Al-Cd, Al-In and Al-Pb binary alloys; (b) effects of various ternary additions such as Mg, Ge and Si on heterogenous nucleation of solidification of Cd and Pb solidification by Al; (c) heterogenous nucleation of solidification of Si by solid Al in hypoeutectic Al-Si alloys. In addition, the melting behaviour of Cd, In and Pb particles embedded in an Al matrix is investigated. The rapidly solidified microstructures of melt spun Al-Cd, Al-In and Al-Pb alloys consist of faceted 5-200nm diameter Cd, In and Pb particles homogeneously distributed throughout an Al matrix. Cd particles exhibit an orientation relationship with the Al matrix which can be described as {111}_Al//{0001}_Cd and andlt;110andgt;_Al//andlt;112and#773;0andgt;_Cd, and In and Pb particles exhibit a near cube-cube and cube-cube orientation relationship with the Al matrix respectively. Cd, In and Pb particles embedded in the Al matrix exhibit distorted truncated octahedral or truncated octahedral shapes surrounded by {111}_Al and {100}_Al facets. The solid Al-solid Cd, solid Al-solid In surface energy anisotropies are constant over the temperature range between room temperature and Cd and In melting points respectively. The solid Al-liquid Cd and solid Al-liquid In surface energy anisotropies decrease with increasing temperature above Cd and In melting points. Solidification of Cd, In, Pb particles embedded in an Al matrix is nucleated catalytically by the surrounding Al matrix on the {111}_Al faceted surfaces with an undercooling of 56, 13 and 22K and a contact angle of 42°, 27° and 21° for Cd, In and Pb particles respectively. Addition of Mg to Cd particles embedded in Al increases the lattice disregistry across the nucleating plane, but decreases the undercooling before the onset of Cd(Mg) particle solidification. Addition of Ge to Al decreases the lattice disregistry across the nucleating plane, but increases the undercooling before the onset of Pb particle solidification embedded in the Al(Ge) matrix. These results indicate that chemical interactions dominate over structural factors in determining the catalytic efficiency of nucleation solification in Al-Cd-Mg and Al-Pb-Ge alloys. Contact between Si precipitates and Pb particles embedded in an Al matrix decreases the undercooling before the onset of Pb particle solidification. The equilibrium melting point of Cd particle in the melt spun Al-Cd alloy is depressed because of capillarity, and the depression of equilibrium melting point increases with decreasing particle size. In the melt spun Al-In and Al-Pb alloys, however, most of the In and Pb particles embedded within the Al matrix grains are superheated, and the superheating increases with decreasing particle size. The heterogeneous nucleation temperature for Si solidification by Al depends sensitively on the purity of the Al. Na and Sr additions have different effects on the Si nucleation temperatures. With an Al purity of 99.995%, Na addition increases the Si nucleation temperature, while Sr addition does not affect or decreases the Si nucleation undercooling, depending on the amount of Sr addition. The solidified microstructure of liquid Al-Si eutectic droplets embedded in an Al matrix is affected by the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle, however, with high Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form a large number of non-faceted Si particles embedded in Al.

edited by N. J. Grant and B. C. Giessen. Section I. / Includes bibliographical references and index.

Powder bed fusion (PBF) is a metal additive manufacturing process that is increasingly used in the aerospace and medical industry to build complex parts directly from computer-aided design. Due to the presence of large temperature gradients and rapid cooling rates during the processing, the PBF process is assumed to follow a rapid solidification processing route. However, the extent of deviation of the solid-liquid interface from equilibrium as a function of processing conditions has not been studied in detail for the PBF process. In this thesis, a numerical model is developed to study the interfacial characteristics as a function of processing conditions to characterize if the PBF process exhibits rapid solidification or not. The model is based on the work of Hunt et al. [1, 2, 3] and is capable of simulating cellular and dendritic growth at both low and high interface velocities. The developed model accounts for the various undercooling such as constitutional and curvature undercooling, the variation of the liquidus temperature with composition, and the partition coefficient and diffusion coefficient with temperature. Moreover, the variation of the partition coefficient and the liquidus slope with the growth velocity has also been considered in the developed model. The model is used to predict the range of primary cellular/dendritic spacing for a given set of input parameters. In addition to this, the tip undercooling, tip Péclet number and spacing Péclet numbers have also been estimated using the model to quantify the extent of deviation of the solid-liquid interface from equilibrium. A good qualitative agreement between the predicted values from the numerical model and the analytical KGT model is achieved. This new model can be used to understand the relationship between the processing conditions, material system and interfacial characteristics during the PBF process, and thus improve microstructural development during PBF processing. / Thesis / Master of Science in Materials Science and Engineering (MSMSE)

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